In reciprocating engines, a technique for bringing rollers provided on a skirt portion of the piston into rolling contact with the inner surface of the side wall of the cylinder is proposed in order to decrease the sliding frictional resistance between the inner surface of the side wall of the cylinder and the side surface of the piston through piston rings and the like in the reciprocation of the piston.
According to the technique for involving such rollers, the weight of the piston increases and the inertia of the piston during reciprocation increases. As a result, the responsiveness of the engine is reduced. For example, driving performance such as acceleration performance is deteriorated, and hence driving becomes difficult.
In view of the above, it is an object of the present invention to provide an engine for decreasing the frictional resistance in the piston reciprocation by floating the lateral-pressure receiving side of the piston from the inner surface of the side wall of the cylinder by using a gas chamber which is provided between the side surface of the piston and the inner surface of the side wall of the cylinder without increasing the weight of the piston.
The other object of the present invention is to provide an engine with a simple structure and high durability by floating the lateral pressure receiving side of the piston from the inner surface of the side wall of the cylinder by using the gas pressure of a gas chamber into which the gas pressure is introduced from the combustion chamber, the gas chamber being formed between the inner surface of the side wall of the cylinder and the side surface of the piston opposite to the inner surface of the side wall of the cylinder.